1. Field of Invention
The invention relates to a method for polishing a copper layer on a workpiece, and particularly to polishing a copper layer on a workpiece such as a semi-conductor wafer where it is important to polish the copper layer in accordance with a predetermined polish profile.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is now a commonly used technique for planarizing thin films of copper deposited onto a workpiece such as a semiconductor wafer to form damascene and dual damascene interconnect wires and vias. The method can also be applied to other workpieces, such as flat panel displays. In the chemical mechanical polishing process, the surface of the workpiece is subjected to both abrasion and chemical treatment through the use of a chemical slurry in conjunction with polishing motion between a polishing pad and the workpiece surface. Another polishing technique does not employ a chemical slurry, but incorporates abrasive particles into a xe2x80x9cfixed abrasivexe2x80x9d material used to fabricate the polishing pad. Still another technique for polishing copper films is electropolishing.
Briefly, in CMP, the workpiece to be polished is first placed in a carrier (or holder), where it is held immovably with the workpiece surface to be polished facing a polishing pad that has a surface area substantially greater than the workpiece surface. A chemically reactive slurry is supplied either through channels in the polishing pad, or directly onto the workpiece surface so that it enters into the interface between the polishing pad and the surface. To achieve polishing, movement may be imparted to the carrier, the polishing pad, or both. These movements include rotational, orbital, and linear movement (in one or more directions).
One of the recurring problems associated with CMP is the tendency to produce regions that are over-polished, and other regions that are under-polished. This kind of xe2x80x9cdifferential polishingxe2x80x9d is particularly common in areas adjacent to the perimeter of the workpiece surface, especially when polishing electrodeposited copper thin films.
Electrodeposited copper thin films on semiconductor wafers tend to have areas of thicker deposition. Typical film thickness deposited for performing damascene or dual damascene copper wafers is on the order of 1 micron. In certain areas of the wafer, the particular features of the semiconductor die patterns forming the vias and interconnect trenches may cause the deposition of copper to be more rapid than in less structured areas of the die. Typical variation seen is on the order of an additional 0.5 microns thickness. A copper CMP process can normally process semiconductor wafers with this type of thickness variation and produce acceptable product. However, an additional feature in electroplated (EP) copper wafers may be present on the wafer. The electrode or set of electrodes contact a very thin seed layer of copper that had been deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) on top of a diffusion barrier layer during the early parts of forming a damascene structure. During electroplating of the copper film, some ring style electrodes may leave a thicker peripheral ring or xe2x80x9cskirtxe2x80x9d of copper on the outer top surface. Some multiple point contact electrodes may leave not only a local thicker area around each electrode but also leave a void directly under the point of contact, creating a small raised structure surrounding a very thin deposit xe2x80x9cvolcanoxe2x80x9d or xe2x80x9cscallopedxe2x80x9d structure in the film deposition. Unfortunately, for the typical 1 micron copper film deposition, the peripheral ring or skirt on the outer top surface of the workpiece may have up to an additional 1 micron of material deposited due to the electrode structures and the process of the deposition.
The extra copper material deposited due to the electrodes makes copper CMP particularly difficult. The thinner areas of the film are cleared before the other peripheral areas. Polishing long enough to clear the outer thicker areas usually results in excessive dielectric erosion and metal thinning in the other parts of the workpiece.
In general, CMP apparatus provide a compromise between a high rate of material removal and an acceptable variation in final film thickness, when semiconductor wafers are being polished. It is generally thought that a uniform removal rate, in accordance with a predetermined polish profile, will achieve a planar surface. However, as previously described, many thin films, especially those formed of the newer copper technology, are not deposited on the surface of the workpiece in a uniform manner. In these films, it is frequently necessary to remove material from the surface in accordance with the material that was not uniformly deposited. Conventional CMP processes, designed to polish a copper layer to a predetermined polish profile, on the assumption that thin films were somewhat uniformly laid down, experience difficulties with the removal of non-uniform thin films. For example, a CMP process may be tailored for xe2x80x9ccenter fastxe2x80x9d material removal or xe2x80x9ccenter slowxe2x80x9d removal depending upon the generally expected thickness distribution of the thin film material on the workpiece surface. Such a CMP process does not completely compensate for the actual variations in thickness in localized areas on the workpiece surface.
The invention provides a method and apparatus for polishing a surface of a workpiece, whereon there is deposited a copper or other metallic pattern, such as a film, on at least a portion of the surface, in a manner which addresses issues related to the non uniform deposition of the metal onto the workpiece surface.
According to the present invention, local regions of the metal pattern such as a film or layer, which would present problems in achieving a desired surface planarity or curvature using a predetermined polish profile, are selectively polished before the metal layer is polished in accordance with the predetermined polish profile in order to achieve a closer file with the desired planarity or curvature. For example, where a copper layer has local regions that are extra thick, or has a xe2x80x9cvolcanoxe2x80x9d or scalloped configuration (e.g. at the periphery of the layer), these local regions are first selectively polished to reduce their thickness to such an extent that these regions more closely match the surrounding workpiece surface regions, before the overall surface is polished in accordance with the predetermined polish profile. Similarly, if local regions of the copper layer have other surface characteristics which would make the entire copper layer more difficult to polish in one step in accordance with a predetermined polish profile, then local regions are first selectively polished, to bring them to a state where the local regions of the copper layer and the rest of the surface can then be effectively polished in accordance with the predetermined polish profile.
While reference may be made to copper as an example of the metal patterns contemplated, clearly other metal patterns also benefited by application of the invention.